Measuring instruments that use an electrochemical cell as are typically provided by a disposable test strip or the like are well known and popular with consumers. These instruments are used for the detection of various analyte levels in physiological fluid samples. For example, the concentration of an analyte in a variety of different physiological samples, such as urine, tears, saliva, and the like can be determined with these instruments. One popular application is for determining the concentration of an analyte in interstitial fluid, blood or blood fractions, and more particularly in whole blood.
A typical test strip includes a fluid sample application area that includes an electrochemical cell having a reagent integrated with a pair of electrodes that can be electrically connected to a measuring instrument. In making a measurement, a small volume of a physiological fluid such as blood is applied to the sample application area so it wets the reagent. The measuring instrument applies a potential across the electrodes and the fluid chemically reacts with the reagent in a way that changes a measurable electrical property of the reaction fluid (its conductivity, for example) that can be correlated to a concentration of the analyte of interest. As a result, an electrical characteristic of the reaction fluid (typically its ability to conduct a current) can be measured with a suitable electronic system of the measuring instrument. The measured electrical characteristic is related to the concentration of the particular analyte being measured in the physiological fluid and can be used to determine the concentration of the analyte. For example, in a blood glucose measurement, an oxidation current that is produced can be measured and used to determine glucose concentration in the blood sample.
Typically, the magnitude of the measured current is integrated over a predetermined time period and used to determine a concentration value for the analyte being measured. Because of this, it is important to measure the current after it is known that a sufficient amount of the sample to be measured is present. Thus, many measuring instruments include a way to check for the presence of a fluid sample before initiating measurements that are used to determine analyte concentration.
One way to check for the presence of a fluid sample on a test strip is to apply a potential to the electrodes of the test strip while the measuring instrument is waiting for application of a sample. The presence of a continuous current for a predetermined period of time and above a predetermined magnitude indicates the presence of sufficient sample, which indication can then be used to trigger the start of a measurement cycle. If a short duration current is measured (less than the predetermined period of time), the measuring instrument determines insufficient sample exists and an error condition is generated. While this technique is effective, an extraneous signal or event can lead to such an error condition even when no sample at all is present (with a dry test strip, for example) or when sufficient sample is present. As one example of such an event, electrostatic discharge can, under certain circumstances, provide a false signal. Electrostatic discharge typically provides a short duration current with high voltage within an electrode that may be detected. Thus, it is desirable for the measuring device to be able to distinguish between a short duration current caused by the presence of insufficient sample and to properly indicate a true error condition and a short duration current caused by an extraneous event such as electrostatic discharge.
Electronic hardware devices can be used to filter or suppress extraneous signals such as those caused by electrostatic discharge. However, such hardware based devices are typically non-adjustably configured to provide a specific filtering or suppression function. For example, many hardware filters are designed to simply block a current spike having a predetermined magnitude anytime it occurs and do not have the ability to consider other factors such as when or how long the current spike occurs.